I had an inadvertent opportunity to stage a multi-year field trial with jalapeno chiles over a five-year period, and my experience indicated that plant stress after the chiles have set fruit is a huge determining factor in how hot the fruit will be. Contributors to stress are heat, lack of root space, water variations, and consistent high wind (!). I've been growing chiles for several decades, and by far the hottest jalapenos I've ever grown were those that were cultivated in third-floor window boxes on deck railings with a western exposure, meaning that they got late unobstructed late afternoon sun for several hours, and regular high wind. Jalapenos from the same cell-pack/same reputable nurseries (Chalet and Gethsemane in alternating years), were grown in large terracotta pots, on an east-facing deck, with less late-afternoon sun and wind (meaning more water retention). Soil/slow-release fertilizer were identical. The window-box chiles were so hot as to be nearly inedible, particularly when red-ripe; also, the striations (AKA "corking") were
far more prevalent on this group than on the east deck group. The results were consistently the same every summer from 1997 through 2002. Interestingly, the same stress doesn't seem to make poblanos noticiably hotter - it only produces smaller chiles and blossom-end rot, just as similar stressors trigger it in tomatoes.
I have been growing my jalapenos in-ground and in similarly-sized large pots since moving from that properly, and have found no significant heat level differences in the chiles. Seems the combo of high wind, root stress from lack of space, and fast water evaporation from excessive sun exposure are the main stress variables in producing incendiary jalapeno chiles.
Regarding buying reliably hot jalapeno chiles, the best way to go seems to be to patronize Mexican markets, and buy red jalapenos, selecting the corkiest ones available; Marketplace on Oakton carries
jalapenos rojos on occasion, as well. I usually buy mine from Michoacan market, on Western Ave. just north of Touhy, which almost always has them in stock.